Significance The development of an effective contraceptive vaccine could be useful in countries where medical care is limited. Objectives The goal of this study is to test the concept of contraception via immunization against sperm and semen-specific antigens. In phase 1, 3 groups of 8 females each will be immunized with different preparations of sperm and seminal proteins along with a control group of 8 placed on a similar schedule receiving vehicle only. Blood will be collected once prior to the immunization, then once every 2 weeks following each immunization treatment in order to assess the antibody levels. A vaginal wash sample will be collected 3 days before the first breeding is scheduled in order to assess the localized antibody concentration. Following the immunization schedule, a series of timed matings will be scheduled over 4 consecutive menstrual cycles. The pregnancy rate will determine the actual success of the different treatment groups in comparison to the control group. All pregnant animals will be removed from the study. In phase 2, there will be a single treatment group of 12 ani mals that receive LDH-TT, along with a control group of 12 animals receiving a synthetic peptide without LDH. These animals will undergo similar immunization and mating schedule as in phase 1. Results Animals in phase 1 are receiving their final booster and will be placed onto a mating schedule. Future Directions If contraception can be achieved using unpurified antigens such as these being tested, then a more purified antigen such as LDH-C4 will be studied. KEY WORDS immuno-contraception, LDH-C4, sperm antigens, vaccine FUNDING NIH Grant HD29125